Question: What material is used in a gravitational wave and what is the effect of it?
Paraphrase: What is a gravitational wave made of and what are it's effects?
SPARQL (Wikidata): SELECT ?ans_1 ?ans_2 WHERE { wd:Q190035 wdt:P186 ?ans_1 . wd:Q190035 wdt:P1542 ?ans_2 }
Answer: spacetime, gravitational wave event